Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for performing a marine seismic survey using autonomous underwater vehicles (AUVs) that carry appropriate seismic sensors.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure beneath the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of the geophysical structures under the seafloor is an ongoing process.
Reflection seismology is a method of geophysical exploration to determine the properties of earth's subsurface, which is especially helpful in the oil and gas industry. Marine reflection seismology uses a controlled source of energy that sends the energy into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
A traditional system for generating seismic waves and recording their reflections off the geological structures present in the subsurface is illustrated in FIG. 1. A vessel 100 tows an array of seismic receivers 110 provided on streamers 112. Streamers may be disposed horizontally, i.e., lying at a constant depth relative to the ocean surface 114. The streamers may have spatial arrangements other than horizontal. Vessel 100 also tows a seismic source array 116 configured to generate a seismic wave 118. Seismic wave 118 propagates downward and penetrates the seafloor 120 until a reflecting structure 122 (reflector) eventually reflects the seismic wave. Reflected seismic wave 124 propagates upward until it is detected by the receiver(s) 110 on streamer(s) 112. Based on the data collected by receiver(s) 110, an image of the subsurface is generated by further analyses of the collected data. Seismic source array 116 includes plural individual source elements, which may be distributed in various patterns, e.g., circular, linear, at various depths in the water.
However, this traditional configuration is expensive because the cost of streamers is high. New technologies deploy plural seismic sensors on the bottom of the ocean (ocean bottom stations) to improve the coupling. Even so, positioning seismic sensors remains a challenge.
Newer technologies use autonomous underwater vehicles (AUVs) that have a propulsion system and are programmed to move to desired positions and record seismic data. After recording the seismic data, the AUVs are instructed to return to a vessel or underwater base to recharge their batteries and/or transfer the seismic data. Various methods have been proposed for deploying and collecting the AUVs. However, none of the existing methods fully address the needs of a seismic survey that uses AUVs which land on the ocean bottom to collect the seismic data.
Accordingly, it would be desirable to provide systems and methods that provide an inexpensive and efficient method for deploying AUVs on the ocean bottom, to record seismic waves, and resurface after recording the data.